Motors on VFD's

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Cody K

Senior Member
Location
Texas
Does anyone now a general rule on how slow a vfd can power an induction motor without overheating or damaging the motor itself? We use vfd's on cooling fans and we usually limit their speed to at least 20%. We had a situation where recently the cutler hammer vfd was shutting the motor down due to overheating (somehow the vfd sensed this). The speed limit was not installed so I am assuming that is the reason for this. Thanks
 

mdshunk

Senior Member
Location
Right here.
I don't know what the lower limit rule of thumb is, but I clearly remember having to install an auxillary cooling fan kit on an air compressor motor that was VFD driven because it often operated too slowly to cool itself.
 
We run AC motors from fractional HP to 300HP on VFD's, we do not have to limit minimum speed, and have no problems, as long as the V/f curve is correct, but as Mark said, they do require auxilary cooling as the motor mounted fan is not effective at low speeds.
 

eric stromberg

Senior Member
Location
Texas
Check with the manufacturer. Many manufacturers have a chart which shows the slowest speed with the motor as is. By installing a larger fan, the motor can be run at lower speeds. Sometimes, in order to install a larger fan, a shaft adapter must also be installed. Somewhere in my files i have all this information for Marathon and, i think, Reliance. It's a few years old but it would give you the idea. I'll post it sometime next week.

Also, if you are running the motor in a classified area, you will have to verify that the temperature at slower speeds will still be appropriate for the atmosphere.
 

tony_psuee

Senior Member
Location
PA/MD
Cody,

The motor should have a rated turn down ration, 10:1, 6:1, etc. That would indicate the minimum speed. Most of the VFD's I have dealt with, Eaton in particular, the motor over temperature is a calculated value, not measured. So, the motor may not actually be overheating. I suggest you record the volts, current, Hz, readings at shutdown and contact your Eaton rep.

Tony
 

eric stromberg

Senior Member
Location
Texas
As i mentioned earlier, this file is a few years old. Model numbers may not be the same. Check with the manufacturer for your specific application.
 

bsh

Senior Member
Section 430.126 discusses thermal protection requirements for a motor operating with a VFD. 430.126(A)(2) states that a VFD must have load and speed sensitive protection and also provide thermal memory protection when it is shut down. If the VFD provides the thermal protection for the motor and does not use the methods described in (1), (3), or (4) then it must also have a memory so if it is shut down it will remember the motor has been running and has some heat built up and protect the motor accordingly. This was a new requirement in 2005 and not all VFD's have the provision to monitor motor heat
 

cschmid

Senior Member
I use VFD's on fans and the heating issue is mathematical calculated or you could use an infrared temp gun and get an accurate temp and then do the math..the VFD has a Temp sensor built into it..You will need the owners manual to do this one.. but if the motor is undersized or the pulleys have been change to adjust fan speed it will cause the motor to work harder which increases heating so problem could be as easy as changing pulleys on the fan...
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
You realize of course that at 20% speed the likelihood that you are still moving a significant amount of air is fairly low (assuming it is a centrifugal fan), therefore the load on the motor is almost nil. If you haven't already done it, put the VFD in V/Hz^2 (Variable Torque) mode. Most VFD manufacturers will tweak their motor protection scheme when they know that the lower the speed, the less torque is required. If you left the drive in Constant Torque mode it would "ass-u-me" that you were trying to run a fully loaded conveyor at that speed.
 
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